1. Field of the Invention
The invention generally relates to current source inverters for induction motor control, and more particularly, to an adjustable speed polyphase induction motor drive.
2. Description of the Prior Art
Current source inverters for induction motor control are well known in the prior art. See, for example, the patent to Mittag, U.S. Pat. No. 1,946,292. Polyphase inverters, as exemplified by the Mittag patent, are forced commutated inverters of the capacitive coupled type, employing blocking diodes to trap peak voltage on the capacitors. This concept has been employed very seldom during the succeeding years primarily because of the high cost, poor efficiency, and low reliability of thyratron switches.
Prior art current source inverters customarily have a polyphase diode bridge rectifier connected to the motor terminals to serve as a clamp during inverter commutations. The D.C. side of this rectifier has an electrolytic capacitor bank to accept the energy contained in the inductive component of motor current, and a resistor or other means to bleed off this energy between commutations.
Specifically, the leakage reactance field energy is transferred to the clamp upon commutation of motor current from one line to another.
Conventional current source inverters also have the disadvantage of low power factor when supplying an induction motor. This is due to the fact that total output current must be drawn from the input lines on a one-to-one basis. In these prior art systems D.C. input current to the inverter is controlled by phase control of a controlled bridge rectifier connected on its D.C. side to the inverter via a D.C. inductor. At no load, the magnetizing current of the motor is drawn from the input lines at zero power factor, and throughout the range of load and speed of variable frequency operation, all lagging or reactive KVA at the load is drawn from the input lines as such.
Another problem inherent in conventional current source inverters is that the ripple voltage present on the output of the controlled bridge rectifier periodically adds and subtracts from the ripple generated on the D.C. link by the inverter. This creates beats and large ripple at low frequency when the variable frequency output is close to the line frequency or a multiple of the line frequency. This large, low frequency ripple current creates torque disturbances and affects motor stability adversely.